#app4 .circle{
    border: 1px solid black;
    margin-top: 3vw;
    margin-left: 3vw;
    width: 10vw;
    height: 10vw;
    border-radius: 50%;
}

@keyframes change{
    0%{
        background-color: pink;
    }
    100%{
        background-color: blueviolet;
    }
}

#app4 .circle.active{
    animation: change 1s linear infinite alternate;
}